Monoclonal antibodies (MAbs) were used to study the diversity of cytochromes P-450 in human tissues such as placenta and lymphocytes. The MAbs were used to study P-450 dependent aryl hydrocarbon hydroxylase (AHH) and ethoxycoumarin deethylase (ECD) and determine the contribution of epitope specific P-450s to each reaction in each tissue studied. This approach can be used as the basis for a general method for phenotyping the diversity of P-450s in human tissues.